A vinyl ether is conventionally produced by the Reppe reaction, in which an alcohol and acetylene are reacted in the presence of an alkali metal catalyst at a high temperature of 120 to 200° C. The Reppe reaction proceeds quickly and is an effective means for producing a vinyl ether when the starting alcohol is rich in reactivity such as a primary or secondary alcohol. However, there is the problem that, for an alcohol having a low reactivity such as a tertiary alcohol, the alcohol conversion is not sufficient and, further, the vinylization does not proceed completely (Non-Patent Literatures 1 and 2).
Further, the method for reacting the alcohol and acetylene in a so-called super basic medium has been reported. For example, the method for using an alkali metal hydroxide, as a catalyst, and performing the reaction in an aprotic polar solvent to efficiently produce a vinyl ether has been reported (Non-Patent Literature 3). However, while this method is fast in reaction and excellent in alcohol conversion rate and vinyl ether production rate in the case of a primary and secondary alcohols, it has the problem of a low yield in the case of vinylization of a tertiary alcohol.
Furthermore, an improved method for reacting an alcohol with acetylene in a super basic medium has also been reported. That is, the method for accelerating the reaction by forming anhydrous cesium hydroxide in the reaction system and using it as a catalyst has been reported (Non-Patent Literature 4). However, in this method as well, when performing the reaction under an acetylene initial pressure of 16 atm, the conversion rate of the tertiary alcohol, i.e., t-butanol is 25%, while the yield of the vinyl ether is 8%, i.e., insufficient. Further, when performing the reaction under an acetylene pressure of atmospheric pressure, it is also described that not even a trace amount of t-butylvinyl ether can be detected.
In addition, it is reported that, after a catalyst is prepared by reacting a tertiary alcohol, i.e., 1-adamantanol and tri(n-propyl)carbinol with metal potassium in a toluene solvent the vinylization is carried out under a 18 kg/cm2 acetylene initial pressure (Non-Patent Literature 5). However, in this method as well, despite the reaction being carried out under a high acetylene pressure, the yield is a low as 67 to 40%, and therefore the method is not said to be sufficient.
Furthermore, the method for obtaining the desired vinyl ether by using a relatively easily obtainable alkylvinyl ether from an ether exchange reaction of alcohol in the presence of a metal catalyst is known in the art. For example, the method for reacting n-butylvinyl ether and the tertiary alcohol, i.e., t-butanol and 1-adamantanol in the presence of a palladium catalyst stable in the air is reported (Non-Patent Literature 6). However, there are the problems with this method that the yield is a low as 72 to 61%, a long time of 32 to 48 hours is required for equalization, an amount of alkylvinyl ether used is 20 times of the alcohol and thus the productivity is of low.
As explained above, in the conventional method for producing vinyl ether, when the vinylization of alcohol is insufficient, separation by distillation is not efficient, because vinyl ether and alcohol have similar structures. For this reason, production of high purity vinyl ether causes a high load on the purification process and features a low yield, and therefore the problem is a high production cost.
As a solution, an efficient vinylization method of alcohol having a low reactivity is proposed. Patent Literature 1 discloses a method for reacting an alcohol with acetylenes in a liquid phase in the presence of a co-catalyst such as a basic alkali metal compound and 1,4-diethoxybutane or 1,4-divinyloxybutane. However, to achieve a sufficient conversion of the alcohol by this method in a short time, as disclosed in the examples, a high acetylene pressure of about 20 kg/cm2 (absolute pressure) is required. Acetylene is extremely unstable under a pressure exceeding 2 kg/cm2 (gauge pressure) and a self decomposition and explosion easily occurs. The higher the pressure, the higher the risk. The above method is not preferable in terms of safety. Further, Patent Literature 1 describes use of phenol as a starting material and synthesis of phenylvinyl ether in an N-methyl-2-pyrrolidone solvent, but does not describe in any way the use of a tertiary solid alcohol as a starting material.
As another method, the method for reacting a vinyl ester compound and alcohol in the presence of a metal catalyst to obtain the corresponding vinyl ether is known. For example, the method for using an iridium compound, as a transition metal catalyst, and for reacting vinyl acetate and 1-adamantanol in a toluene solvent in the presence of sodium carbonate by an ester exchange reaction to obtain a desired product is disclosed (Patent Literature 2). In this method, the conversion rate of 1-adamantanol reaches 93% and the yield of the desired product is also as high as 91%. However, in addition to unconverted alcohol, about 1% of unnecessary by-product is formed. This method is not said to be sufficient for obtaining a high purity product. Furthermore, an expensive iridium catalyst is used, the reaction system is dilute, the productivity is low, and other issues remain to be solved.